In an automatic door of a building or an electric slide door apparatus for a vehicle such as a wagon or a van, an electric opening/closing apparatus is provided for sliding a door by a motor or the like to open/close the door. In the electric opening/closing apparatus like this, there is a possibility that an object may be sandwiched between a door frame and the door at the time of a closing operation of the door (an entrapment state). An electric opening/closing apparatus including a control means for detecting an entrapment to stop the closing operation of the door or to reverse the closing operation to the opening operation of the door in such a situation is suggested.
For example, an object detecting sensor, which detects an entrapment with use of a piezoelectric element for detecting acceleration generated by shock or vibrations, is being investigated and is gradually put to practical use. The piezoelectric element generates electric polarization corresponding to external force (stress) such as acceleration. A piezoelectric sensor, in which the piezoelectric element is utilized, outputs oscillating voltage signals, such as a sine wave, by effect of electric polarization generated corresponding to external force induced by an entrapment, or the like, of an object by an opening/closing door, or the like. The sensor outputs voltage signals, not only on the basis of an entrapment, but also on the basis of mechanical vibrations. However, phenomena to be detected (in this case, an entrapment of an object) and other mechanical vibrations can be preferably distinguished because of difference in a frequency of a voltage waveform therebetween. Further, the sensor generates a voltage even when the external force is small, for example, at an initial stage of an entrapment. Accordingly, the sensor can detect an entrapment even at an early stage. Therefore, countermeasure against an entrapment can be taken before force applied to an entrapped object becomes large. JP2003-106048A2 (Patent document 1) describes a technique for using the object detecting sensor, in which the piezoelectric element is utilized, in a hatch back door of an automotive.
An object detecting apparatus described in the Patent document 1 includes, as illustrated in FIG. 17, a piezoelectric sensor 1, a filter 40 for passing a predetermined frequency component of an output signal from the piezoelectric sensor 1, and a judging portion 50 for judging an entrapment on the basis of the output signal from the filter 40. Characteristics of the filter are opted considering vibration characteristics, or the like, of a vehicle body of the automotive. Specifically, the filter is a low-pass filter that extracts signal components equal to or lower than approximately 10 Hz, in order for removing vibrations caused by an engine of the automotive or vibrations caused by running of the automotive. The judging portion 50 is a comparator. If amplitude of the output signal transmitted from the piezoelectric sensor 1 through the filter is equal to or larger than a predetermined value (TH50), the comparator judges that an entrapment has occurred. Reference number 70 in FIG. 17 indicates an operational amplifier for performing impedance conversion, signal amplification, or the like.
As described above, the piezoelectric sensor utilizes piezoelectric effect, in which voltage is generated on the basis of deformation of the piezoelectric element, the deformation induced by vibrations, or the like. Accordingly, the piezoelectric sensor outputs detection signals, not only on the basis of vibrations induced by shock of an entrapment, or the like, but also on the basis of vibrations induced by other phenomena. For example, in a situation where the piezoelectric sensor is provided at a slide door of a vehicle, the piezoelectric sensor outputs detection signals even on the basis of vibrations of the running vehicle, on the basis of vibrations of an engine, or the like. In a waveform illustrated in FIG. 18A, a center part of the waveform has large amplitude. Such a waveform having large amplitude is induced by an entrapment. A right and left part of the waveform has smaller amplitude. Such a waveform having small amplitude is generated by vibrations of the vehicle and by vibrations of the engine. As illustrated in FIG. 18A, frequency of the output of the piezoelectric sensor caused by these mechanical vibrations is high, which is different from a situation where an object is entrapped. Accordingly, the output of the piezoelectric sensor caused by these mechanical vibrations can be preferably removed by the filter 40 (low-pass filter) illustrated in FIG. 17. Further, even if the output of the piezoelectric sensor caused by these mechanical vibrations is not sufficiently attenuated by the filter 40 and a part thereof passes through the filter 40, because amplitude of the part passing through the filter 40 is small, the amplitude does not exceed a judging threshold TH50 in the judging portion 50. Accordingly, the output of the piezoelectric sensor caused by these mechanical vibrations is not wrongly detected as an entrapment.
However, vibrations caused by each phenomenon do not necessarily have clear difference from vibrations caused by other phenomenon. For example, an opening/closing operation of an electric slide door apparatus for a vehicle is not only a simple sliding operation. While the door is being closed, after the slide door slides toward a front direction of the vehicle, the slide door is pulled from the outside of the vehicle to the inside of the vehicle along a bending portion of a door rail (introducing operation). If the slide door of the vehicle slides in a uniform direction, acceleration is not generated. However, in a situation where the slide door starts to slide along the bending portion (introducing operation), a moving direction of the slide door drastically changes. Accordingly, acceleration is generated. When the slide door starts to slide along the bending portion (introducing operation), as illustrated in FIG. 18B, voltage generated by the piezoelectric sensor drastically rises, and after that, the output of the piezoelectric sensor becomes a waveform like a gradual discharge of stored charge. There is a possibility that a discharging time Tb of this discharge becomes close to a half period of time Ta of an output waveform of the sensor in a situation where an entrapment has occurred (illustrated in FIG. 18A). Accordingly, there is a possibility that the output of the piezoelectric sensor passes through the filter 40. Further, because amplitude of the output of the piezoelectric sensor is sufficiently large, there is a possibility that the output of the piezoelectric sensor exceeds the judging threshold TH50 in the judging portion 50, and the output of the piezoelectric sensor is detected as an entrapment. There is a possibility that such a phenomenon occurs, not only on the basis of a change of a sliding direction of the slide door, but also on the basis of external noise.
Further, even in a situation where an entrapment actually occurs, for example, as illustrated in FIG. 18C, there is a possibility that amplitude of the output of the piezoelectric sensor is small according to softness of a material of an entrapped object. A period (2Tc) of the waveform of the output of the piezoelectric sensor is similar to a period (2Ta) of the center part of the waveform illustrated in FIG. 18A. Accordingly, the output of the piezoelectric sensor passes through the filter 40. However, because amplitude of the output of the piezoelectric sensor is small, the amplitude of the output of the piezoelectric sensor does not exceed the judging threshold TH50 in the judging portion 50. Thus, there is a possibility that a correct judgment cannot be made only on the basis of evaluation of frequency and strength (amplitude) of the signal, according to a conventional configuration illustrated in FIG. 17.
For overcoming this, an option would be to employ a method for accurately analyzing a frequency, instead of utilizing the filter 40. However, as described above, frequency of the output voltage of the piezoelectric sensor is very low, precisely, equal to or lower than approximately 10 Hz, in a situation where an entrapment is detected. Accordingly, if the waveform of the output voltage of the piezoelectric sensor is monitored for one cycle for purposes of judgment of the frequency, it takes an inverse number of the frequency, in other words, equal to or longer than 100 milliseconds. By doing so, advantage of utilizing the highly sensitive piezoelectric sensor, by which an initial stage of an entrapment can be detected, cannot be sufficiently exerted.
A need thus exists for an object detecting apparatus, which can preferably detect an entrapment by analyzing an output of a sensor for a short time without mixing up a detection signal generated by a phenomenon different from an entrapment with a detection signal generated by an entrapment. The present invention has been made in view of the above circumstances and provides such an object detecting apparatus.